This is probably the best Indian restaurant that I have ever visited. Having never been to India, I can't comment much on authenticity, but I will say that the vegetarian options are much more interesting than you find at most Indian restaurants in the US: mango mushrooms, Bombay soya nuggets, eggplant tamarind... New Delhi also has a page of more "standard" vegetarian dishes on the menu, like channa masala and navrattan curry.
Another perk is the palatial decor - tall, arched ceilings supported by ornamented columns. The walls are adorned with poetry, paintings and traditional costumes, with plaques explaining their historical significance. The prices are fairly high: $15 for an entree, and rice is NOT included (you can purchase a bowl of rice for an extra $4). However, I found the cost to be reasonable for a delicious meal and fascinating history lesson.